1. Field of the Invention
The present invention relates to (i) a novel ester compound, (ii) a polymer comprising units of the ester compound which is blended as a base resin to formulate a resist composition having a high sensitivity and resolution, and in particular, suitable as micropatterning material for VLSI fabrication, (iii) a resist composition comprising the polymer, and (iv) a patterning process using the resist composition.
2. Prior Art
While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. In particular, photolithography using a KrF or ArF excimer laser as the light source is strongly desired to reach the practical level as the micropatterning technique capable of achieving a feature size of 0.3 xcexcm or less.
For resist materials for use with a KrF excimer lasers, polyhydroxystyrene having a practical level of transmittance and etching resistance is, in fact, a standard base resin. For resist materials for use with ArF excimer lasers, polyacrylic or polymethacrylic acid derivatives and polymers containing aliphatic cyclic compounds in the backbone are under investigation. All these polymers have advantages and disadvantages, and none of them have been established as the standard base resin.
More particularly, resist compositions using derivatives of polyacrylic or polymethacrylic acid give relatively satisfactory results with respect to resolution, but have extremely low etching resistance and are impractical because the resin backbone is weak. On the other hand, resist compositions using polymers containing alicyclic compounds in their backbone have a practically acceptable level of etching resistance because the resin backbone is robust, but are very low in resolution because the reactivity of acid-decomposable protective groups is extremely low as compared with those on the acrylic polymers. They are impractical as well. While a finer pattern rule is being demanded, there is a need to have a highly transparent resin which has a high reactivity enough to provide a satisfactory resolution, and at the same time, a robustness enough to provide a sufficient etching resistance.
Therefore, an object of the present invention is to provide (i) a novel ester compound capable of forming an acid-decomposable, robust polymer, (ii) a polymer which is blended as a base resin to formulate a resist composition having a higher sensitivity, resolution and etching resistance than conventional resist compositions, (iii) a resist composition comprising the polymer as a base resin, and (iv) a patterning process using the resist composition.
The inventor has found that a novel ester compound of the following general formula (1) obtained by the method to be described later is useful in preparing an acid-decomposable, robust polymer; that a resist composition comprising as the base resin a novel polymer prepared from the ester compound to a weight average molecular weight of 1,000 to 500,000 has a high sensitivity, resolution and etching resistance; and that this resist composition lends itself to precise micropatterning.
In a first aspect, the invention provides an ester compound of the following general formula (1). 
Herein R1 is hydrogen, methyl or CH2CO2R3, R2 is hydrogen, methyl or CO2R3, R3 is a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, R4 is a branched or cyclic tertiary alkyl group of 5 to 20 carbon atoms, Z is a divalent hydrocarbon group of 1 to 10 carbon atoms, and k is 0 or 1.
In a second aspect, the invention provides a polymer comprising recurring units of the following general formula (1a-1) or (1a-2) derived from an ester compound of the general formula (1), and having a weight average molecular weight of 1,000 to 500,000. 
Herein R1, R2, R4, Z and k are as defined above.
In a preferred embodiment, the polymer further comprises recurring units of at least one of the following formulas (2a) to (10a). 
Herein R1, R2 and k are as defined above; R5 is hydrogen or a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms; at least one of R6 to R9 is a carboxyl or hydroxyl-containing monovalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R6 to R9, taken together, may form a ring, and when they form a ring, at least one of R6 to R9 is a carboxyl or hydroxyl-containing divalent hydrocarbon group of 1 to 15 carbon atoms, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R10 is a monovalent hydrocarbon group of 3 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure; at least one of R11 to R14 is a monovalent hydrocarbon group of 2 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure, and the remainders are independently hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms, or R11 to R14, taken together, may form a ring, and when they form a ring, at least one of R11 to R14 is a divalent hydrocarbon group of 1 to 15 carbon atoms containing a xe2x80x94CO2xe2x80x94 partial structure, and the remainders are independently a single bond or a straight, branched or cyclic alkylene group of 1 to 15 carbon atoms; R15 is a polycyclic hydrocarbon group of 7 to 15 carbon atoms or an alkyl group containing such a polycyclic hydrocarbon group; R16 is an acid labile group; X is xe2x80x94CH2xe2x80x94 or xe2x80x94Oxe2x80x94; and Y is xe2x80x94Oxe2x80x94 or xe2x80x94(NR17)xe2x80x94 wherein R17 is hydrogen or a straight, branched or cyclic alkyl group of 1 to 15 carbon atoms.
In a third aspect, the invention provides a resist composition comprising the polymer defined above, and more specifically, a resist composition comprising the polymer defined above, a photoacid generator, and an organic solvent.
In a fourth aspect, the invention provides a process for forming a resist pattern comprising the steps of applying the resist composition defined above onto a substrate to form a coating; heat treating the coating and then exposing it to high-energy radiation or electron beams through a photo mask; and optionally heat treating the exposed coating and developing it with a developer.
The polymer comprising recurring units of formula (1a-1) or (1a-2) originating from the ester compound of formula (1) is robust since it contains a bridged aliphatic ring in its backbone. The polymer is significantly improved in reactivity as well for the following reason.
As described above, the polymer containing an alicyclic compound in its backbone has the general tendency that the reactivity of acid-eliminatable protective groups is low. However, the inventors have found that when an acid-eliminatable site is located at a position spaced from the backbone of the polymer, the reactivity is outstandingly improved possibly because of the increased probability of contact with acid. Then the polymer comprising recurring units of formula (1a-1) or (1a-2) has an acid elimination reactivity surpassing prior art polymers comprising recurring units of formula (0a-1) or (0a-2) below. Then a resist composition comprising the polymer as a base resin possesses both a high etching resistance due to the robustness of the resin and a satisfactory resolution due to the high reactivity of acid-decomposable groups and is thus quite useful in micropatterning. 
Herein, R1, R2, R4 and k are as defined above.